Conventional diffusion flame combustors for gas turbines typically comprise a fuel injector unit mounted at one end of a flame tube that delimits a combustion chamber. The other end of the tube is connected to a duct for conveying combustion gas to the turbine blades. The flame tube is mounted coaxially within a tubular container that is in communication with a combustion air compressor discharge casing and, together with the flame tube, defines a space for combustion air, or air space. The injector unit is mounted so as to have its head inside the combustion chamber and is provided, in a two-fuel version, for example, with an axial atomizer nozzle for the liquid fuel and, around it, an annular injector nozzle or a crown of nozzles for injection of the gaseous fuel. The air arriving from the air space is conveyed into the combustion chamber independently of the injector unit. Holes appropriately distributed along the flame tube are provided for input of primary, secondary and dilution air.
Although useful, a simple-cycle gas turbine engine of this type, which is presently used throughout the electric power industry to accommodate peak energy generation needs, often has emission levels too high for those pollution standards or limits of many power generation locations and, therefore, can be subjected to operating power limitations or prohibitions at many sites. Worldwide there are thousands of gas turbine engines in operation for providing peak load services. While typically the gas and oil firing NOx (nitrogen oxides) emissions produced, at maximum load engine conditions, are approximately 0.17 and 0.3 Kg/Mcal, respectively, such emission levels are frequently above those permitted by law. Moreover, in many countries, average NOx emission are below about 0.13 Kg/Mcal; and in the next few years a further reduction in emission standards to about 0.065 Kg/Mcal or even lower is anticipated. Accordingly, it has usually been necessary to average the NOx emissions produced by gas turbine engines with other sources operating well below the system-wide emission limits in order to achieve compliance.
In a gas turbine combustor, NOx emissions are essentially generated by two mechanisms:                1) Primary mechanism: namely, by fixation of atmospheric nitrogen in the flame (thermal NOx);        2) Secondary mechanism: that is, by conversion of nitrogen chemically bound in the fuel (chemical NOx), as in some lower quality heavy fuel oils, process gases and some coal gases from gasifiers with hot gas clean up.        
According to the Zeldovich mechanism, the rate at which thermal NOx is formed increases exponentially with the flame temperature and linearly with the time the combustion gas remains at the flame temperature. Consequently, the peak flame temperature and the retention time are the principal variables that control NOx formation and the resulting emission levels. Furthermore, the rate at which nitrogen oxides are formed diminishes rapidly when the flame becomes poor in fuel and as the peak temperature diminishes. Therefore, introduction of small quantities of diluents into the primary combustion zone has the effect of reducing the rate of thermal NOx formation.
Consequently, to maintain old gas turbine engines in operation when more stringent emission limits are required, the following possible options must be considered:                a) installing water or steam injection systems on gas turbine engines to achieve between 40% and 50% NOx reduction of actual emission values. This option requires additional equipment and increases operating costs due to loss of engine efficiency caused by the relatively high water/fuel ratios utilized;        b) installing sophisticated and relatively expensive dry low-NOx combustion systems that also require considerable modifications to the engine control equipment, if such are based on multistage premixed combustion processes;        c) retrofitting combustion systems with minor modifications for achieving both low NOx emission levels and low impact on the engines.        
In order to make emissions of gas turbine machines that are fired with natural gas compliant with current emission standards, General Electric - Power Systems (GE-PS) recently developed low NOx emission versions of such a combustor for their MS-5000 gas turbines, an industrial-type gas turbine with combustors of the canannular type. Without making use of DLN technology, the solution proposed by GE-PS, it has been found, reduces NOx emissions by varying the distribution of air within the combustor by modifying the sizes and patterns of holes in the flame tube (and, thereby, increasing primary air flow) in such a way as to assure lean mixing ratios in the primary combustion zone at maximum load.
This solution, which merely calls for physical modifications to the flame tube, has the advantage of being relatively easy to realize and implement during the course of ordinary gas turbine maintenance. However, because temperature peaks in the primary combustion zone are attenuated, thereby, reducing the reaction rate in the diffusion flame, it can also cause increased production of unburned or un-combusted matter. In addition, since the geometry of the combustion system remains relatively constant, it is not possible for the retention time to be increased.